The perl style is considered a bad idea, because it can become a no-op, if the value being passed in is 'undef'. Here's how:

Let's say you have a perl-style accessor $self->foo(), which is used to access the value $self->{foo}.

$self->{foo} is currently eq 'bar'.

A caller comes along with a variable $_, and wants to set the foo value to whatever's in $_. They therefore call $self->foo($_).

The bug: if $_ is undef, that means that $self->foo(undef) is called. In a perl-style accessor, that is considered a 'get' operation instead of a 'set', so after that call, $self->{foo} is still set to 'bar'.

In other words, it's impossible to use a perl-style accessor to set a value to 'undef', and it's easy to accidentally perform a no-op instead of a set. This has bitten us in the past.

In the Java-style accessor, the source code itself mandates whether the operation is a set or a get; the data cannot affect which operation happens. Hence, it's safer.